Infection remains the most frequent illness of man and one of the most common causes of death. The investigations proposed here seek to gain understanding of the physiology of host defense against infection, especially bacterial and fungal infection. The ultimate goals of this research are the more effective prevention and management of infections in the normal and compromised host and the eventual manipulation of the function of the immune system to enhance its protective effects. The focus of these studies is the oxygen-dependent mechanisms by which phagocytic cells (neutrophils, monocytes, and macrophages) kill microorganisms. In particular, the projects proposed here seek to gain knowledge about the molecular nature of the events that comprise the "respiratory burst system": binding of the stimulus (organism) to the phagocyte membrane Greater than initiation and transduction of a biochemical signal Greater than triggering of the enzyme responsible for converting oxygen to its microbicidal metabolites. Most of the projects explore these overlapping events as they occur in the macrophage, in which they have been incompletely studied. In addition, the biochemical events under scrutiny will be compared in normal and "activated" macrophages, which are capable of markedly increased activity of the respiratory burst. Thus, comparative analyses of the components of the system will attempt to determine the molecular basis for this critically important aspect of macrophage activation. Plasma membrane-related projects include analysis of surface membrane glycoproteins, determination of the effects of modifying macrophage membrane lipids on activity of the respiratory burst, and quantification of receptor number and affinity for the most effective stimulus of the macrophage respiratory response. Membrane Greater than transduction projects include study of the relationships between activity of the respiratory burst and changes in membrane potential and calcium ion mobilization in normal and activated macrophages. In projects related to the respiratory burst enzyme, kinetic parameters of the enzyme in the two macrophage types will be compared, using enzyme functioning as a component of membrane fragments and in purified form; and the functional role of divalent cations in the catalytic activity of this enzyme in neutrophils and macrophages will be determined. The acquisition of the activated state through exposure to synthetic platelet-activating factor or endotoxin, and the loss of the activated state during culture will also be analyzed. Improved understanding of the mechanisms responsible for the potent oxidation response of phagocytic cells might permit its eventual pharmacologic modulation.